Thermal production of astrophobic axions

Understand

Hot axions are produced in the early Universe via their interactions with Standard Model particles, contributing to dark radiation commonly parameterized as $\Delta N_{\text{eff}}$.

• In standard QCD axion benchmark models, this contribution to $\Delta N_{\text{eff}}$ is negligible after taking into account astrophysical limits such as the SN1987A bound.
• We therefore compute the axion contribution to $\Delta N_{\text{eff}}$ in so-called astrophobic axion models characterized by strongly suppressed axion couplings to nucleons and electrons, in which astrophysical constraints are relaxed and $\Delta N_{\text{eff}}$ may be sizable.
• We also construct new astrophobic models in which axion couplings to photons and/or muons are suppressed as well, allowing for axion masses as large as few eV.